-endothelial tube approach and fuse with each other to form an endothelial tube network endothelial tube network appears in chorionic membrane and body stalk, and connect to vitelline circulation 原始血管 原始血细胞---endothelial tube approach and fuse with each other to form an endothelial tube network ---endothelial tube network appears in chorionic membrane and body stalk, and connect to vitelline circulation